ELI5- how do rice cookers know how long to cook the rice for no matter the different quantities
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[This](https://youtu.be/RSTNhvDGbYI?si=Jjq-1kyO9oPFqB2M) is an amazing video that explains your question perfectly.
If there’s still liquid water in the pot, the max temperature is 212F/100C. When it runs out of water, then the temperature can climb past 212F. Put in a switch (bimetallic strip) to turn off the machine when it hits 215-220ish (or whatever the margin of error is) and you have an extremely simple, purely mechanical system that will run until the water boils off then turn off when it starts to heat beyond boiling.
Same principle for the simplest Coffee Machines or Kettles that go until they run out of water.
Technology Connections made a perfect video answering this exact question: https://youtu.be/RSTNhvDGbYI?si=phn5j6VFh6bR6qqF
So if I wanted to get a better rice cooker, how much would I need to spend? Looking at Amazon (for info) there are models from about $110 up to several hundred dollars.
They don’t “know”, they just boil the rice until its done.
When is it done? When there’s no liquid water left. Once all the water present has either been absorbed or vaporized, then the cooker assumes that the rice is done (and assuming you put in the right proportions of rice and water, it will be).
So, how does it know when there’s no liquid water left. That’s the part that might not be intuitive.
Boiling water at sea level remains at 100 degrees centrigrade. At higher altitudes (meaning lower air pressure), the boiling temperature is lower, but under normal conditions, it’s not going to get higher. If you put more heat into boiling water, it doesn’t heat up, it just boils faster. Therefore, if you’re continually heating a pot and it stays at 100 degrees centigrade, you can pretty much assume it has water boiling in there. But once all the liquid water is gone, there can be no more boiling, so, if you continue to add heat, the temperature starts rising again.
What rice cookers do is continually heat the pot at a certain wattage, which means that the water steadily heats up, then steadily boils. Once the temperature in the pot goes above 100 degrees centrigrade, it automatically shuts off the main boiling cycle (and typically shifts to a lower-power warming mode.
You can design an automatic switch that uses a thermocouple and programming and such, but traditional rice cookers use a simpler design. They use a switch with a permanent magnet, and a disc made of an iron alloy. The disc is held in place by the magnet, keeping a circuit closed and running the cooker, but if you heat up the disc hot enough, it will stop being attracted to the magnet (this is known as the “Curie point”). The alloy is specifically designed to have a Curie point a little higher than 100 degrees centigrade. Hence, when the water all boils away, the temperature rises, the magnetic switch opens, and the cooking circuit is broken. Simple, reliable and inexpensive. All the things engineers love.
The premise is a bit wrong, the short eli5 is it depends and some really do have just a stupid timer.
Most basic ones that use more than a timer will just stop heating (or turn into keep warm mode) when no more water is present. It is either detected by a sharp rise in temperature, finally being able to go above the boiling point of water, or using something a bit more fancy like steam detection.
The really advanced ones however have more complex logic. One reason is that they need it because they use a much heavier cooking vessel like a fat cast iron pan, and if you did that you’d end up burning the rice on the bottom (just try it, even if you remove it from the heat it stays hot for a while), but it is used to give even heating to the whole bowl of rice (instead of only a heating element on the bottom that barely gets the tiny sides hot). They can measure how much rice you put in and let you select between different modes, some keep pressure high for faster cooking. The fancy ones can get quite expensive and obviously most people are not going to feel like spending as much as an iPhone on a rice cooker, and nobody is going to do teardowns to show how they work cause it’s a lot harder to reverse engineer.
🤦 this is how you answer this for a 5 year old y’all..
the boiling water keeps some piece cold enough for it it to keep the heat on, and once the waters gone or absorbed into the rice, the rising temp triggers an automatic shutoff.
That’s a great question! Most rice cookers have sensors that detect when the water has been absorbed by the rice, which signals the end of the cooking process. So whether you have a large or small quantity of rice, the cooker will adjust the cooking time based on this sensor.
For what it’s worth, rice cookers also work with other whole grains. We use it regularly with buckwheat, quinoa, oats, millet (my favorite) and others.
Wondered this myself, I’m sure there’s different styles but most of them have a thermocouple that basically just determines if it’s above/below the boiling point of water. Once all the water is absorbed/boiled away, the temperature starts to rise and the thermocouple sends a signal to shut off the heat
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